Frequency modulation systems exchange bandwidth for improved noise performance. However as increased receiver noise, due to the wider band IF amplifiers exceeds the FM carrier, rapidly increasing noise impulses are generated. Restricting the noise bandwidth, prior to limiting, reduce the threshold level at which the noise impulses are generated but this increases signal distortions. Normally an FM receiver's bandwidth, prior to limiting, is fixed, usually designed to optimally satisfy signal distortion consistent with the lowest noise threshold level. As carrier to noise ratios (C/N) fall to threshold, due to received signal fades or equipment deterioration, the output signal to noise falls even more rapidly and there is advantage to adjusting the design trade-off between distortion and noise.
Wideband satellite earth stations, which employ wide deviation FM and threshold extension feedback loops, cannot fully exploit the full threshold extension theoretically possible because of excess delay in the feedback loops which causes oscillation. However, as the noise bandwidth defining filter is narrowed, the excess delay becomes less significant and the feedback factor can be increased. As the feedback factor is increased, the distortion created by restricting noise bandwidth filters is reduced. The feedback factor can theoretically be increased to the point where loop oscillations are triggered. These oscillations normally occur above the video band. A method that can extend the noise threshold of FM receivers to obtain increased fade margin will be useful in improving the practicality of many radio transmission systems and particularly satellite earth stations.